The present invention relates generally to a low-noise multi-output power supply circuit and a method of designing a low-noise multi-output power supply circuit featuring efficient linear regulators. The power supply circuit and method provides a small saturable reactor core placed in series with selected AC output windings of the multi-output power supply to subtract a small amount of average voltage (volt-seconds) from each of the selected windings. This allows different rectified DC voltages to be obtained from different secondary windings even though the different secondary windings have the same number of turns (a single turn in most cases).
To achieve high efficiency in a multi-output, low-noise power supply that employs a multiple number of linear regulators, it is desirable that the voltage dropped across each linear regulating element be as small as possible. Due to the high frequency and low output voltages required, only a single secondary turn (or in some cases 2 turns) is normally needed. This results in high xe2x80x9cgranularityxe2x80x9d so that slightly different output voltages cannot be obtained. For example, if the number of secondary turns is increased from 1 to 2, the output voltage doubles, often requiring a very large increase in the voltage drop across the linear regulator and hence a very large increase in the linear regulator power dissipation.
On the other hand, there needs to be at least a few hundred millivolts voltage drop across each linear regulator so that the regulator can do its job. If one power supply secondary output voltage is regulated in a conventional feedback fashion (without a linear regulator), then the other output voltages (which may be required to be regulated to the same level) will not have a sufficient xe2x80x9coverheadxe2x80x9d voltage to operate properly. It would be desirable to obtain slightly different transformer secondary output voltages without changing the number of transformer turns. This would allow all of the linear regulators to operate with a minimum overhead voltage and therefore operate with high efficiency.
Multiple output voltage, low noise power supplies have typically employed individual primary circuits and transformers, one for each different output voltage power supply. Alternatively, one transformer with multiple secondary windings has been used and the granularity has been accepted, along with the high overhead voltage and high inefficiencies associated with this approach. Both of these approaches lead to increased complexity and costs. Tight magnetic coupling between the transformer secondaries along with coupling of the output filter inductor windings typically cannot be achieved to the degree necessary to meet the voltage regulation requirements for multiple high power outputs.
A low-noise multi-output power supply circuit and method of design thereof features efficient linear regulators, and provides a small saturable reactor core placed in series with selected AC output windings of the multi-output power supply to subtract a small amount of average voltage (volt-seconds) from each of the selected windings. This allows different rectified DC voltages to be obtained from different secondary windings even though the different secondary windings have the same number of turns (a single turn in most cases). This overcomes the xe2x80x9cgranularityxe2x80x9d problem of having a low number of turns on the secondary windings (e.g., going from 1 to 2 turns doubles the output voltage), and reduces the voltage drop across the series regulating element of each linear regulator used in the low-noise, low-voltage outputs and thereby increases their efficiency. Different cross-sectional area reactor cores subtract different amounts of voltage since they introduce different amounts of delay in the reactor core saturation time and hence a different amount of voltage xe2x80x9choldoffxe2x80x9d to the output circuits. Using this technique, a single primary power circuit can be used to supply multiple linear regulators with minimum overhead voltages which are not constrained by the granularity imposed by the low number of turns in the secondaries, to provide an inexpensive, efficient, multiple output, low noise, and relatively high voltage power supply.
A low-noise multi-output power supply circuit and method of design thereof features efficient linear regulators and uses a single primary circuit that provides a minimum overhead voltage to all linear regulators (resulting in high efficiency). Saturable reactors, which do not require another separate winding for control, reduce the average voltage supplied to the regulators, each of which is set using an appropriate cross-sectional core area, providing an effective fractional transformer turn variation using a saturable core, and resulting in a reduction in the granularity limitations normally encountered in multiple secondary turns with a low number of secondary turns.